This invention relates to the fabrication of monolithic articles using aromatic heterocyclic polymers and copolymers.
Rod-like aromatic heterocyclic polymers, as well as copolymers, are widely described in the literature. They have the potential for replacing the state-of-the-art structural materials. These polymers have repeating units of the general formula -Bz-Ar-, wherein Bz is a benzbisazole group of the formula: ##STR1## wherein X is -S-, -O- or -NH-; and wherein Ar is a para-oriented aromatic group such as ##STR2## or the like, including substituted moieties thereof. For example, poly(p-phenylene benzobisthiazole) (PBT) has superior mechanical properties and thermal stability. These polymers and copolymers are linear, and in the art are called rod-like, because the right-most bond of the repeating unit is parallel, and preferably uniaxial, with the left-most bond, regardless of the rotational aspect of the components of the unit.
However, the rigid-rod polymers and copolymers do not melt and do not possess a glass transition temperature, thus limiting their processing capability. They dissolve in strong acids and are normally processed in the solution state. They are recovered from the solution state by coagulating the polymer or copolymer with water. Inasmuch as the mixing of acid and water is exothermic, the heat generated causes void formation. Thus, the polymers and copolymers can only be processed into thin films or fibers. Bulk coagulation is difficult, if not impossible, because of void formation.
Chopped fiber reinforced plastics are currently being used in the fabrication of a wide variety of components. There are several disadvantages in the use of fiber for the reinforcement of plastic. In the case of chopped glass fibers, a large amount of fiber, generally a minimum of 30 percent by weight, is necessary for reinforcement because of the low reinforcing effect of the fiber. There is a practical processing limit on the effective fiber length. A macroscopically long fiber length is required with due regard for breaking or destruction of the fiber during processing, particularly molding.
Composite materials containing chopped fibers are generally less processable than their non-reinforced counterparts. The shape of moldings is often limited to simple block or sheet forms. Films or filaments cannot be formed from chopped glass fiber-reinforced plastics. Other disadvantages of these materials include poor surface properties of molded articles, an anisotropy in dynamic properties, molding defects due to heterogeniety of the polymeric materials, and low cycle time in processing.
A need exists for high strength reinforced composites and a method for their manufacture which possess at least the following desirable prerequisites: (1) non-reliance on fiber reinforcement for the attainment of high strength properties; (2) circumvention of the complexities of current composite fabrication procedures; and (3) elimination of any possibility of fiber-polymer interface problems.
Various attempts have been made to overcome some of the above-described disadvantages of chopped-fiber reinforced plastics. One approach described by Helminiak et al, U.S. Pat. Nos. 4,207,407 and 4,377,546, comprises the dispersion of an intrinsically rigid rod-like heterocyclic polymer in a flexible, coil-like heterocyclic polymer. The result is termed a molecular composite.
The flexible coil polymers referred to above can be benzazole polymers having repeating units of the formula: ##STR3## wherein X is as described above, or benzbisazole polymers having repeating units of the general formula -Bz- Ar'-, wherein Bz is as described above and Ar' is a non-para-oriented aromatic group such as: ##STR4## or the like, including substituted moieties thereof.
While the molecular composite approach represents a valuable contribution to the art, it has certain drawbacks. As noted previously, poly(p-phenylene benzobisthiazole) (PBT) has superior mechanical properties and thermal stability. However, PBT degrades before it melts; therefore, processing of a composite containing PBT must be carried out in a solution state with an acid, such as methanesulfonic acid (MSA), as the solvent. Relatively few flexible coil polymers can be dissolved in or are stable in MSA, thus limiting the choice of matrix polymers. Molecular composites based on PBT and poly-2,5-benzimidazole (ABPBI) have been fabricated into fibers and thin films. However, ABPBI does not have a glass transition temperature (T.sub.g). Therefore, molecular composites containing ABPBI are difficult to thermally consolidate into thicker specimens. To overcome this problem, thermoplastic matrices have been used so that the molecular composite films could be laminated. One approach described by Arnold et al, U.S. Pat. No. 4,977,223, comprises the dispersion of a rigid-rod aromatic heterocyclic polymer in a mixture of a thermoplastic polymer and a thermosetting polymer. However, thicker specimens fabricated using thermoplastic matrices are limited to use at temperatures below the T.sub.g of the matrix polymer(s). Conventional thermoset resins, such as bismaleimides, epoxies and the like, are not stable in the acid medium used to process the rigid-rod polymer, and cannot be used as host matrices for molecular composites.
Another drawback to molecular composites has to do with the propensity of the rod-like materials to agglomerate. Serious agglomeration can lead to structural failure.
Accordingly, it is an object of this invention to provide a novel method for producing monolithic articles using rigid-rod polymers.
It is another object of this invention to provide a method for producing monolithic articles using a molecular composite.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.